1. Technical Field
This invention relates to a semiconductor laser device to be suitably used for a light source of an optical telecommunication system or an optical data processing system.
2. Prior Art
As is well known, semiconductor laser devices realized by using III-V group compounds, InGaAsP-InP semiconductor laser devices in particular, play a vital role in modern optical telecommunication and data processing systems.
For instance, a III-V group compound semiconductor laser device comprising an active layer for laser oscillation surrounded by a heterojunction interface and a p-type substrate has remarkable advantages as described below.
Firstly, since a pnp interface having a large pressure resistance for the pn junction is used for the buried blocking layers of the device, the device will have an output capacity higher than that of a device comprising an n-type substrate and is operational at high temperature.
Secondly, since its p-type ohmic electrode is formed on a P-type semiconductor substrate containing impurities at a higher level, their contact resistance is rather low, making the device a low energy consuming one.
FIG. 2 of the accompanying drawings illustrates a buried type semiconductor laser device comprising a p-InP substrates as disclosed in Laid Open Japanese Patent Application No. 61-190993.
This known semiconductor laser device may be prepared typically by a process as described below.
In the first crystal growth step, a p-InP clad layer 22, an InGaAs active layer 23 and another p-InP clad layer 24 are sequentially formed on the (100) surface of p-InP substrate 21.
In the following etching step, the upper surface of the p-InP clad layer 22 is covered with a 1.5 to 2.0 m wide dielectric mask along a center line of the clad layer 22 and thereafter the p-InP clad layer 22 is etched down to the p-InP substrate 21 except the masked area to produce a straight and narrow mesa.
In the second crystal growth step, a p-InP buried layer 25 is formed on each lateral side of the mesa, which is then sequentially covered by an n-InP buried layer 26 and a p-InP layer 27.
Then, the dielectric mask is removed.
In a third crystal growth step, another n-InP buried layer 28 is formed to cover the upper surface of the mesa as well as the p-InP buried layer 27 located on the lateral sides of the mesa, followed by an n-InP contact layer 29 formed thereon to produce a flat and smooth upper surface for the device.
Then, a p-side electrode 30 and an n-side electrode 31 are formed respectively on the lower surface of the substrate 21 and the upper surface of the n-InP contact layer 29.
Thus, a buried type semiconductor laser device as illustrated in FIG. 2 comprises a p-InP buried layer 25 formed on the lateral sides of the mesa and having a large electric resistance.
The pnpn layer arrangement of the semiconductor laser device yields so many blocking layers that reduce the leakage current.
A known semiconductor laser device of FIG. 2 can, however, show an increased rate of leakage current and hence a high oscillation threshold current if the p-InP buried layer 25 is made thick between the lateral sides of the mesa and the n-InP buried layer 26 as the layer 25 provides a path for the leakage current.
On the other hand, it is very difficult for a semiconductor laser device having a configuration as illustrated in FIG. 2 to exactly show a given distance between the lateral sides of the mesa and the n-InP buried layer 26 because the growth of the thickness of the p-InP buried layer 25 needs to be controlled as a function of the height of the mesa. It is noted that the oscillation threshold current of such a semiconductor laser device can be indefinable particularly when the layers are formed by a liquid phase epitaxial growth technique.
Additionally, if the mesa of the device of FIG. 2 is low, the n-InP buried layer 26 can easily become higher than the mesa such that the n-InP buried layer 26 can eventually reach the n-InP buried layer 28 in the third round of crystal growth to provide a broad way for the leakage current, which by turn can destructively damage the device.
In view of the above described technological problems of existing semiconductor laser devices, it is therefore an object of the present invention to provide a semiconductor laser device that can effectively suppress the leakage current as well as a method for manufacturing such reliable semiconductor laser devices with a high yield and at low cost.